1. Technical Field of the Invention
The present invention relates to a semiconductor laser apparatus for use in various optical communication systems and further relates to a method for modulating the semiconductor laser.
2. Description of the Related Art
Semiconductor lasers are frequently used as light-emitting sources in optical communication systems. Modulation methods are basically divided into two types: those for directly modulating a driving current of a semiconductor laser using transmission data, namely, direct modulation methods, and those for indirectly modulating the output light from the semiconductor laser using an external modulator, namely, external modulation methods.
A commonly used prior-art direct modulation system uses a threshold current I.sub.T of a semiconductor laser as a bias current, and superimposes on the bias current a modulation current which is responsive to transmission data, thereby driving the semiconductor laser. In such modulation methods, however, the semiconductor laser must be driven by a pulse current with a relatively large amplitude. This produces chirping (i.e., a dynamic wavelength shift) at an oscillation wavelength, which distorts the wave form of the optical pulse propagating in the optical filter. This causes the problem that it is difficult to conduct direct modulations by using a driving current of high-speed data of several to several tens of Gb/s. Further, as the modulation amplitude is large, a large load is imposed on both the semiconductor laser and the driving circuit thereof.
To decrease such chirping, a constant bias current I.sub.O which is larger by a sufficient margin than the threshold current I.sub.T of the laser, is applied to the modulating current I.sub.m, which is typically .+-.several tens of milliamperes. Thus, a modulation is performed as shown in FIG. 1, using only that current region which is larger than the threshold current I.sub.T of the laser. This method has a big problem in that, even when the transmitting data is `0`, a light emission state is maintained, thereby deteriorating the light extinction ratio of the output light.
Prior-art external modulation methods use various kinds of external modulators, such as those using electro-optical-effect material and acoustic-optical-effect material, those of the waveguide path type, and those of the light-deflection type. Therefore, there is a problem that external modulators produce a large loss upon connection, thus necessitating a complicated structure for high-speed modulation.
The structure of an optical interferometer 10 using a commonly used Mach-Zehnder-type electro-optical modulator as the external modulator of the semiconductor laser, is shown in FIG. 2. A constant light beam is input from a semiconductor laser to a single optical waveguide path 1 at the input side of the interferometer 10. This light is divided into two beams which pass two optical waveguide paths 2 and 3. Thereafter these beams are combined to form a single optical waveguide path 4. Electrodes 5 and 6 are provided to the two optical waveguide paths 2 and 3, and the difference in optical path length is varied by applying a suitable voltage to them, thereby creating a phase difference between the two beams to be combined. Thus, the two light beams are caused to interfere with each other upon recombining, producing an output light which is subject to intensity modulation in accordance with the phase difference between the two beams.
Such an optical interferometer 10, however, has the problem that the actual length of the light path varies with temperature, and that the wavelength of an input light itself varies. This causes the phase difference between the two lights to be combined to vary with time, so that a reference point of an operation (a point of phase bias voltage) for an intensity modulation is unstable with time. An ideal relationship between a phase difference and an output light intensity at the time of an intensity modulation, is shown in FIG. 3A. Namely, when the phase difference alternates between .phi..sub.1 (=2n.pi.) and .phi..sub.2 (=(2n+1).pi.), the output light intensity alternates between "1" and "0". If the phase difference between .phi..sub.1 and .phi..sub.2 is changed to that between .phi..sub.1' and .phi..sub.2', the phase bias deviates as shown in FIG. 3B. Thus, the value of the output light intensity even at the maximum is smaller than "1" and the value of the output light intensity even at the minimum is larger than "0", thereby decreasing the light extinction ratio of the output light.
To solve this problem, it is considered that a deviation in the phase difference is detected in the output light and is corrected to eliminate the deviation. However, as the output light is alternated between "1" and "0" at high speed in accordance with a change in phase difference, it is extremely difficult to directly detect the status of a phase from the output light. Therefore, the variation in the phase difference cannot yet be sufficiently suppressed.